The Zipper Merge

The Zipper Merge, also called the late merge is a method for merging traffic into a reduced number of lanes. It is most effectively used in situations where there is a large amount of traffic.

What does the zipper merge accomplish?

Interesting, the zipper merge has not been found to increase the number of vehicles that pass a point over a given time period. But it does significantly reduce the length of the backup line (by up to 40%) and most importantly reduces the difference in speed between the two lanes, which improves safety.

One problem with the zipper merge is that it goes against what most people think is the polite way to merge into a single lane. In an effort to ease the concerns of your average driver, many states have campaigns to promote the zipper merge and in Belgium drivers are required by law to use the zipper merge! Two U.S. states that have published statements on the zipper merge are Washington and Minnesota.

The Minnesota Department of Transportation actually has a statement regarding the zipper merge. Sue Groth, MnDOT traffic safety and technology director was recently quoted saying “raising awareness for motorists to use the zipper merge in construction zones will help reduce crashes, speeds and congestion.” The zipper merge is even mentioned in the MN state drivers license manual.

The zipper merge code of conduct:

Continue as long as possible on the merging lane;

At about 300 meters before the bottleneck (marked with a traffic sign), adjust to the speed of the vehicles driving on the adjacent lane;

Vehicles driving on the adjacent lane deliberately make room for the merging vehicle;

At about 50 meters before the bottleneck, without braking or disturbance of the created space, the vehicle merges. Thus the merging vehicle and the vehicle behind it can continue their ride.

Their findings are summarized as follows: normal work zones with lane closures, drivers do not merge at any one definite point, thus causing sudden interruption in traffic flow and sometimes higher delay. The dynamic late lane merge system (DLLMS) was used to identify a definite merge point, improve the flow of the congested freeway work zones and reduce queue lengths in the freeway travel lanes. During the 2006 construction season, the DLLMS was implemented on three freeway segments in southern Michigan. Each work zone segment involved a lane closure from two to one lane. Based on the travel time characteristics, queue, merge locations, and throughput, the effectiveness of the DLLMS was evaluated by the Wayne State University Transportation Research Group. Before period data was not available, so a conventional work zone merge system located on EB I-94 was used as a control site for the WB I-94 test site. Since the two I-69 test sites are approximately 150 miles away from the EB I-94 control site, the I-94 control site could not be used as a control for the I-69 sites. When comparing the I-94 control and test sites, the presence of the DLLMS improved the flow of traffic and increased the percentage of merging vehicles that merge.